Address review nits: base_ifd sidecar lookup + test invariant (#2315)

* `_geotags.py`: when a base IFD is also tracked in `sidecar_origin`
  (uncommon: a file with no full-resolution IFD on the base side),
  resolve its georef against the sidecar's bytes too.  Mirrors the
  lookup applied to the selected IFD.
* `test_sidecar_own_geokeys_2315.py`: assert the first existing IFD
  tag id is > 254 before inserting the NewSubfileType entry at
  position 0, so a future writer change cannot silently break the
  fixture's "insert at front" assumption.
* `_cog_http.py`: leave a cross-reference comment noting that the
  `sidecar_origin` routing added in #2315 is not threaded through
  the HTTP / fsspec metadata path yet; that gap is tracked by the
  separate HTTP sidecar-byte-order finding.

Author: brendancol

xrspatial/geotiff/_cog_http.py

@@ -239,6 +239,13 @@ def _parse_cog_http_meta(
     # IFD lives in the sidecar; that mirrors the eager local reader,
     # whose sidecar IFDs typically carry no out-of-line geokeys and
     # inherit from level-0 (which sits in the base buffer). #2239.
+    #
+    # The ``sidecar_origin`` kwarg added in #2315 for the eager local /
+    # fsspec paths is intentionally not threaded here. A separate fix
+    # is tracked in the HTTP / dask sidecar-byte-order finding (see
+    # the linked issue / PR for the HTTP side). When that lands, this
+    # call should pick up the same mapping so an HTTP sidecar with
+    # its own geokeys is parsed against the sidecar bytes too.
     geo_info = extract_geo_info_with_overview_inheritance(
         ifd, ifds, header_bytes, header.byte_order,
         allow_rotated=allow_rotated)

xrspatial/geotiff/_geotags.py

@@ -1225,7 +1225,19 @@ def extract_geo_info_with_overview_inheritance(
     if base_ifd is None:
         return info
 
-    base_info = extract_geo_info(base_ifd, data, byte_order,
+    # Mirror the sidecar-origin routing for ``base_ifd``. The base IFD
+    # normally lives in the base file (``data`` / ``byte_order`` are
+    # correct), but a file with no full-resolution IFD of its own could
+    # land here with ``base_ifd`` resolved out of a sidecar. The lookup
+    # is the same shape as the one applied to ``ifd`` above. See
+    # review nit on #2315.
+    base_data, base_byte_order = data, byte_order
+    if sidecar_origin is not None:
+        base_origin = sidecar_origin.get(id(base_ifd))
+        if base_origin is not None:
+            base_data, base_byte_order = base_origin
+
+    base_info = extract_geo_info(base_ifd, base_data, base_byte_order,
                                  allow_rotated=allow_rotated)
 
     # Inherit the per-IFD metadata that the COG writer emits only on the

xrspatial/geotiff/tests/test_sidecar_own_geokeys_2315.py

@@ -106,7 +106,17 @@ def _mark_first_ifd_as_overview(path):
 
     # The IFD entries are sorted by tag id. 254 sorts before every tag
     # ``to_geotiff`` emits in practice (the smallest the writer uses is
-    # 256 / ImageWidth), so the new entry goes at position 0.
+    # 256 / ImageWidth), so the new entry goes at position 0. Assert
+    # the invariant so a future writer change that emits a tag <= 254
+    # fails this fixture loudly instead of silently producing an
+    # out-of-order IFD that the reader could later reject. (Review nit
+    # on #2315.)
+    if n_entries > 0:
+        first_tag = struct.unpack_from("<H", raw, first_ifd_offset + 2)[0]
+        assert first_tag > NSF, (
+            f"test fixture invariant: first emitted tag must be > {NSF}, "
+            f"got {first_tag}"
+        )
     insert_pos = first_ifd_offset + 2
 
     next_ifd_off_pos = first_ifd_offset + 2 + n_entries * 12